A solar cell is one kind of photoelectric conversion device which, using a photovoltaic effect, directly converts received light into electric power and outputs the electric power. Unlike a conventional power generation system, a power generation system using the solar cell does not need energy conversion to thermal energy or kinetic energy in the process. Therefore, although fuel is consumed when solar cells are produced or set, the solar cells have an advantage in that the amount of greenhouse gas typified by carbon dioxide or of emission gas containing a toxic substance per electric power generated is remarkably smaller than that of an energy source based on fossil fuel. In addition, the energy of light from the sun which reaches the earth for one hour corresponds to energy that is consumed by humans for one year. Materials that are necessary for production of the solar cells are basically abundant, and for example, there are almost infinite reserves of silicon. Solar photovoltaic power generation has a high possibility to meet the world's energy demand and expected as alternative energy to fossil fuel whose reserves are finite.
A photoelectric conversion device with the use of a semiconductor junction such as a p-n junction or a p-i-n junction can be classified into a single junction type which has one semiconductor junction and a multi-junction type which has a plurality of semiconductor junctions. A multi-junction solar cell in which a plurality of semiconductor junctions whose band gaps are different from each other are disposed so as to overlap with each other in a travel direction of light can convert sunlight including light with a wide wavelength range from ultraviolet rays to infrared rays into electric energy with higher conversion efficiency without waste.
As a method for manufacturing a photoelectric conversion device, for example, a method is proposed in which two substrates each having a p-i-n junction (or a p-n junction) face each other and attached to each other such that the substrates are each located on the outer side, whereby a so-called mechanical stack structure is formed (e.g., Patent Document 1). With such a structure being adopted, a photoelectric conversion device which has no limitation of a manufacturing process due to a stack structure and which has high conversion efficiency can be realized.